Method, apparatus and system for synchronizing a cellular communication system to GPS time

ABSTRACT

A ranging receiver ( 10 ) including a cellular module ( 11 ), for providing time according to a positioning system to enable synchronizing a wireless system to the positioning system. The ranging receiver ( 10 ) includes both the cellular module ( 11 ) and a main module ( 12 ) that provides ranging receiver functionality. When the cellular module ( 11 ) receives a time-stamped frame from the wireless system, it sends a trigger signal over a special hardware path to the main module ( 12 ), which then relates the time of trigger receipt to time according to the positioning system by calculating a position-velocity-time (PVT) solution, using a local clock ( 18 ) to get the difference in time between receipt of the trigger and the instant to which the PVT solution applies, and then communicates to the cellular module ( 11 ) when it received the trigger, according to positioning system time.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/016,140 filed Dec. 12, 2001, now U.S. Pat. No. 6,748,202 alsoentitled METHOD, APPARATUS AND SYSTEM FOR SYNCHRONIZING A CELLULARCOMMUNICATION SYSTEM TO GPS TIME.

Reference is made to U.S. application entitled METHOD, APPARATUS ANDSYSTEM FOR GPS TIME SYNCHRONIZATION USING CELLULAR SIGNAL BURSTS, havingapplication Ser. No. 09/777,521 and filed on Feb. 5, 2001, whichapplication is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to navigating using a global positioningsystem (GPS), and in particular to systems for providing assisted-GPSand for providing GPS time recovery, especially in weak signalconditions.

BACKGROUND OF THE INVENTION

The operation of a global positioning system (GPS) receiver ispredicated on the receiver having a precise value for GPS time; withoutsuch a precise value, the ranges (from the receiver to one or more ofthe GPS satellites) computed by the receiver are inaccurate, since theyare determined simply as the difference between the time of transmissionand time of receipt multiplied by the speed of light. GPS navigationtherefore relies on all elements of GPS (including both satellites andGPS earthbound receivers) having a clock synchronized to GPS systemtime, which is an approximate version of so-called coordinated universaltime (UTC). GPS (more specifically, the GPS ground monitoring network)disseminates corrections to each satellite (which uses a high accuracylocal clock) to account for the bias and offset of the satellite clockcompared to GPS system time, and the satellites provide thesecorrections in the navigation message they each broadcast; therefore allsatellites are synchronized. A GPS receiver, on the other hand,determines the offset of its local clock as part of the solution of theGPS receiver position. In doing so, however, to the extent that the GPSlocal clock is significantly out of synchronization, the calculation ofthe GPS receiver position is prolonged. Therefore, it is advantageous,in general, to provide a means of synchronizing a GPS receiver clock toGPS system time.

In sufficiently weak GPS signal conditions, a GPS receiver cannotdetermine GPS time unassisted. In such conditions, either the exact GPStime has to be recovered to carry out positioning, or positioning muststop. To avoid having to halt positioning, time recovery must beassisted, and there are many ways to assist a GPS receiver in carryingout time recovery, one being to deliver the exact GPS time from acellular network, such as a GSM network.

Unfortunately, a standard GSM network (and also a third generationwideband code division multiple access network) is not synchronized toany universal time reference, since cellular communication is not per sea navigation tool and therefore there is no need for a universalsynchronized time in providing cellular communications. The only timesynchronization that is often needed is time slot synchronization, wherea mobile station synchronizes itself to a particular base stationschedule in order to keep its own transmission in its assigned time slotand to pick up the messages from the base station intended for themobile station. Such synchronization is therefore (and need only be)relative (between a mobile station and a base station) as opposed touniversal. To enable deriving a universal time from such a cellularnetwork, new equipment and new messages are needed.

There is a device that provides a GPS/GSM timing relationship (mapping);it is called a Location Measurement Unit (LMU); an LMU can be thought ofas a specialized GPS receiver located at a cellular base station, a GPSreceiver adapted to time-stamp with GPS system time communication signalbursts to mobile stations. An LMU time-stamps with GPS time thecommunication signal bursts from base stations. An LMU provides to a GPSreceiver (a GPS receiver configured to make use of the LMU providedinformation) the help needed in weak signal conditions in constructingGPS time (i.e. in synchronizing with GPS system time). The LMU providesa so-called reference time information element, in which it indicateswhich GSM signal frame, time slot and bit are to be used as a timereference point, according to which for example a mobile station canremove the GSM system delay, and so recover exact GPS time.

For typical GPS receiver positioning accuracy, GPS time must be known towithin ˜10 μs in the receiver. Such accuracy is difficult to achieveusing an LMU-based system for time synchronization, but co-owned U.S.application Ser. No. 09/777,521, filed Feb. 5, 2001, hereby incorporatedby reference, provides a solution. According to the art prior to thatapplication, a GPS receiver is adapted to make use of LMU assistance byincluding a cellular component, in addition to the GPS component, thatresponds to the LMU message (including picking up the actual trigger inthe indicated signal frame, time slot, and bit) (see FIG. 1). The twocomponents communicate via a software messaging layer. There are,however, significant random delays in communicating messages over asoftware messaging layer, i.e. internal delays arising because of use ofthe software messaging layer for various other tasks besides onlyproviding LMU time synchronization information. (Random delays occurinside buses used by a GPS receiver, buses where messages aretransmitted from one software module to another using a dedicatedsoftware messaging architecture. Such delays can be tens of millisecondsin duration.) U.S. application Ser. No. 09/777,521 provides a specialhardware connection (see FIG. 1) between the GPS component and thecellular component that is used to signal to the GPS component theprecise time of arrival of a time-stamped frame indicating an instant oftime having a value according to GPS time that is conveyed by anotherframe to the cellular component and then communicated over the softwaremessaging layer to the GPS component.

Thus, an LMU is used, especially in poor signal conditions, tosynchronize a GPS component to GPS time. In many places, however, basestations are not equipped with an LMU. In such situations, it would beadvantageous if a GPS receiver, including both a GPS component and acellular component, that could synchronize itself to GPS time because ofoperating in favorable signal conditions, could then communicate GPStime to GPS receivers (also including both a cellular component and aGPS component) operating in less favorable signal conditions and inplaces where services from an LMU are not available.

SUMMARY OF THE INVENTION

Accordingly, in first aspect of the invention, an apparatus is provided,comprising: a cellular module, responsive to a portion of a downlinkframe signal conveying a cellular frame provided by a cellularcommunication system; and a main module, responsive to ranging signalsfrom beacons of a positioning system in which time is kept according toa predetermined time reference; the apparatus characterized in that: thecellular module provides a trigger signal indicating the arrival of theportion of the downlink frame signal, and is further responsive to asignal indicating information for relating the instant at which thetrigger signal is provided to a time value according to thepredetermined time reference; the apparatus relates the instant at whichthe trigger signal is provided by the cellular module to a time valueaccording to the predetermined time reference; and the main module usesthe time value for synchronizing into ranging signals from beacons ofthe positioning system; wherein the trigger signal is provided over aconnection between the cellular module and the main module soconstructed that any delay between sending and receiving the triggersignal is either negligible or can be determined.

In accord with the first aspect of the invention, the cellular modulemay provide to a base station of the cellular communication system theinformation for relating to a time value, according to the predeterminedtime reference, the instant at which the trigger signal is provided bythe cellular module to the main module. Further the main module mayprovide a position for the instant when the trigger signal is receivedby the main module, and the cellular module may be responsive to theposition, and may provide the position to the base station. Alsofurther, the apparatus may be provided as part of a system alsocomprising the beacons of the positioning system that provide theranging signals and the base station receiving the information forrelating to a time value, according to the predetermined time reference,the instant at which the trigger signal is provided.

In a second aspect of the invention, a system is provided comprising anapparatus according to the first aspect of the invention, and furthercomprising a cellular base station, for providing the cellular frame.

In a third aspect of the invention, a system is provided comprising anapparatus according to the first aspect of the invention, and furthercomprising the beacons of the positioning system that provide theranging signals.

In a fourth aspect of the invention, a method is provided for having aranging receiver, operative in communication with a positioning systemand also operative in communication with a cellular communicationsystem, provide information to a base station of the cellularcommunication system of use in enabling the base station to synchronizeto time according to the positioning system, the ranging receiverincluding a cellular module adapted for communication with the cellularcommunication system and also including a main module adapted forcommunication with the positioning system, the method characterized by:a step in which the cellular module detects a portion of a downlinkframe signal and provides a trigger signal to the main module via aconnection so constructed that any delay between applying a signal andreceiving the signal is either negligible or can be determined; a stepin which the cellular module receives a signal indicating informationfor relating the instant at which the trigger signal is provided to atime value according to the predetermined time reference; and a step inwhich the apparatus relates the instant at which the trigger signal isprovided by the cellular module to a time value according to thepredetermined time reference.

In accord with the fourth aspect of the invention, the method may alsoinclude a step in which the cellular module provides to the base stationthe information for relating to a time value according to thepositioning system, the instant at which the trigger signal is provided.The method may also include a further step in which the main module alsoprovides a position for the instant when the trigger signal was receivedby the main module, and the cellular module is responsive to theposition, and provides the position to the base station.

In a fifth aspect of the invention, an apparatus is provided comprising:a cellular module, responsive to a portion of a downlink frame signalconveying a cellular frame provided by a cellular communication system;and a main module, responsive to ranging signals from beacons of apositioning system in which time is kept according to a predeterminedtime reference; the apparatus characterized in that: the cellular moduleprovides a trigger signal indicating the arrival of the portion of thedownlink frame signal; and the apparatus relates the instant at whichthe trigger signal is provided by the cellular module to a time valueaccording to the predetermined time reference; wherein the cellularmodule provides to the cellular communication system information aboutthe relationship of the downlink frame signal to a time value accordingto the predetermined time reference.

In accord with the fifth aspect of the invention, the cellular modulemay also provide to a base station of the cellular communication systemthe information for relating to a time value, according to thepredetermined time reference, the instant at which the trigger signal isprovided by the cellular module to the main module. Further, the mainmodule may provide a position for the instant when the trigger signal isreceived by the main module, and the cellular module may be responsiveto the position, and provide the position to the base station. Alsofurther, the apparatus may be provided as part of a system alsocomprising the beacons of the positioning system that provide theranging signals and the base station receiving the information forrelating to a time value, according to the predetermined time reference,the instant at which the trigger signal is provided.

In a sixth aspect of the invention, a system is provided, comprising anapparatus according to the fifth aspect of the invention, and furthercomprising a cellular base station, for providing the cellular frame.

In a seventh aspect of the invention, a system is provided comprising anapparatus according to the fifth aspect of the invention, and furthercomprising the beacons of the positioning system that provide theranging signals.

In an eighth aspect of the invention, a method is provided for having aranging receiver, operative in communication with a positioning systemand also operative in communication with a cellular communicationsystem, provide information to a base station of the cellularcommunication system of use in enabling the base station to synchronizeto time according to the positioning system, the ranging receiverincluding a cellular module adapted for communication with the cellularcommunication system and also including a main module adapted forcommunication with the positioning system, the method characterized by:a step in which the cellular module detects a portion of a downlinkframe signal and provides a corresponding trigger signal to the mainmodule via a connection so constructed that any delay between applying asignal and receiving the signal is either negligible or can bedetermined; a step in which the apparatus relates the instant at whichthe trigger signal is provided by the cellular module to a time valueaccording to the predetermined time reference; and a step in which thecellular module provides to the cellular communication systeminformation about the relationship of the downlink frame signal to atime value according to the predetermined time reference.

In accord with the eighth aspect of the invention, the method may alsoinclude a step in which the main module provides a position for theinstant when the trigger signal is received by the main module, and thecellular module is responsive to the position, and provides the positionto the base station.

The invention allows a time stamp to be made in a mobile device, so thatthe mobile device serves the function of an LMU, thus eliminating theneed of having a separate LMU. In addition, the accuracy of theinvention is improved over what is typically provided by an LMU; theaccuracy provided by the invention is for example in the range of 1-2μs.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the inventionwill become apparent from a consideration of the subsequent detaileddescription presented in connection with accompanying drawings, inwhich:

FIG. 1 is a simplified illustration of a GPS receiver including both acellular component and a GPS component with a special hardwareconnection between them, according to the present invention; and

FIG. 2 is a flowchart of a method by which the GPS component of FIG. 1synchronizes to GPS time and then communicates GPS time to the cellularcomponent, which can then communicate GPS time to other GPS receiversadapted as in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 1, a global positioning system (GPS) receiver 10according to the invention is shown as including a cellular module 11(not necessarily providing a complete cellular communication capability)and a GPS module 12. The cellular module responds to a cellularcommunication signal through a cellular antenna 14. The GPS moduleincludes standard GPS hardware 21 for responding to GPS ranging signals(including navigation data) received via a GPS antenna 15 after beingbroadcast by beacons 27 of the positioning system (such as GPSsatellites, but also possibly ground-based beacons). The cellular moduleprocesses cellular communication signals so as to extract timesynchronization information, such as from a Location Measurement Unit(LMU) message, as described above. The cellular communication signalconsists of data organized into frames, including delimiters (headersand footers) enabling the cellular module to distinguish one frame fromthe next. The cellular module includes a frame counter 20 for countingdownlink frames (transmitted by a base station to the cellular module).The frame counter also detects the frame edges, which are used, e.g., infrequency reconstruction. In detecting or identifying a frame edge, theframe counter provides an indication of the instant when a new frame isreceived. In GSM cellular communication, there could also be a time-slotcounter and a bit counter, with corresponding edge detectors.

The GPS receiver 10 also includes a software messaging layer 16, bywhich the cellular module and GPS module communicate messages, butmessages so communicated are subject to random delays in being deliveredbecause of other message traffic being communicated via the softwaremessaging layer (message traffic for example between differentcomponents of the GPS module).

Every time the frame counter indicates the arrival of a new cellularcommunication signal downlink frame (and in particular, a new frameedge), a trigger pulse is generated and communicated to the GPS modulevia a special hardware path 23 having either essentially no delay or adelay that is known and so can be taken into account, i.e. providing thetrigger pulse substantially free of random delay. The trigger pulse isused to latch a register 19 in the GPS module, a register called here atiming register, so that the register stores internal time (not UTCtime), i.e. the time indicated by the GPS receiver oscillator/localclock 18. (The GPS receiver local clock 18 is typically implemented asan internal counter, as part of the GPS hardware. The latching resultsin either storing the current value of the local clock in the registeror resetting the internal counter. The local clock 18 may or may not beactually a part of the GPS module, but is accessible to components ofthe GPS main module even if it is not a part of the GPS main module.)

Still referring to FIG. 1, as explained in co-pending and co-ownedapplication Ser. No. 09/777,521, in places where there is an LMU and inpoor signal conditions where it is helpful for the GPS receiver 10 to beassisted by the base station 24 (hosting an LMU, not shown), when atrigger pulse is generated by a trigger module 16 of the cellular module11, as the trigger pulse is being generated and provided via the specialhardware path 23 to the timing register 19 of the GPS module 12, thecorresponding frame number (or alternatively the time slot or bitnumber, as explained below) is transmitted to the GPS hardware 21 viathe software messaging layer 16. If the GPS hardware has already beenset to GPS time (by a procedure described below), it remainssynchronized by stabilizing the local clock using the trigger pulses,and so countering clock drift. To set GPS time in the GPS module tobegin with, however, i.e. when the GPS receiver is powered on, thecellular model uses a time-stamp it receives from an LMU 22 at a nearbybase station; the time-stamp specifies that an indicated GPS time is tobe associated with receipt of an indicated frame number, time slot andbit number; the frame referenced in the time-stamp is a frame that wasalready transmitted (either broadcast or transmitted point-to-point tothe GPS receiver). The time-stamp includes GPS time associated with theframe, time slot, and bit. (The time stamp does not, however, includepropagation delay. There is no way to know in the LMU, how far the GPSreceiver is from the serving base station where the LMU is located.Propagation delay over the air interface is compensated for in variousways; e.g. in GSM, propagation delay is often compensated for using aso-called Timing Advance (TA) measurement made by the cellular system.TA is measured from a round trip delay between the receiver and theserving base station.)

As also explained in U.S. application Ser. No. 09/777,521, once thecellular module 11 has the time-stamp (value), the cellular module canprepare what is here called a time-mark message (as opposed to atime-stamp message); a time-mark message indicates the GPS time to beassociated with a yet-to-be-received frame. As soon as the frameindicated in the time-mark message is received, a trigger pulse is sentto the GPS module along the special hardware path 23, and the framenumber and time-mark message are communicated to the GPS module via thesoftware messaging layer. The time showing on the local clock 18 whenthe trigger pulse arrives at the GPS module is written into the timingregister 19.

A cellular module measures the propagation delay correction before acommunication channel is opened. A measurement of the propagation delayis one part of the handover process involved in opening a communicationchannel. The propagation delay is thus always known to a cellularmodule.

Now, the problem solved by the invention is in the situation where a GPSreceiver (adapted as in FIG. 1 and so able to communicate with acellular base station) enjoys favorable signal conditions and so candetermine GPS time as a by-product of a position determination (i.e. asan output of a so-called position-velocity-time (PVT) solution), butother GPS receivers in the same coverage area (or even other coverageareas) do not enjoy favorable signal conditions, and there are no LMUsavailable to help synchronize these other GPS receivers to GPS time. Theidea of the invention is to have the GPS receiver 10 that cansynchronize to GPS time (because it has good signal conditions) performa PVT solution based on the ranging signals it receives and sosynchronize itself to GPS time (at least at the instant for which thePVT solution applies), and then signal to the serving base station(through its cellular module 11) an actual (more precise) GPS time valuefor a time-stamped frame provided earlier by the base station, as in theabove description. In other words, the base station provides a(time-stamped) frame (a possible time stamp occurring at a frame edge)and either a time-stamp value or no time-stamp value, and the GPSreceiver adapted as in FIG. 1 and operating as described below, sendsback a message to the base station telling the base station the GPS timevalue for the instant at which the time-stamped frame (the triggeringframe) arrived at the GPS receiver. (In case the base station provides atime-stamped frame having no time stamp value, i.e. for which anotherframe does not provide a time value for the arrival of the time-stampedframe edge, the time-stamped frame serves only as a point of reference,i.e. an instant for which a time value is to be provided by theinvention.) The difference in time between when the time-stamped frameis transmitted by the base station and when it is received can be takeninto account in several different ways (such as by having the basestation do the propagation delay accounting based on the timing advanceit measures for the GPS receiver), none of which are the subject of theinvention; and for purposes of the description here, the propagationdelay is assumed to be negligible.

Referring again to FIG. 1, to understand the invention, it is necessaryto understand some limitations of how the GPS receiver 10 functions.First, frame edges of downlink frames are detected by the rollover ofthe frame counter 20, which synchronizes itself to the incoming dataframes (transmitted by the base station 24); the GPS module has nocontrol over the operation of the frame counter. Triggering occurs onlyfrom the cellular module 11 to the GPS module 12, never in the otherdirection. Uplink data frames (that is, e.g. speech data beingcommunicated by the cellular component 11 to the base station 24) cannotbe GPS-time stamped accurately due to unknown delays in the cellulartransmitter (of the cellular transceiver 25) and protocol stacks(implemented in the cellular processor 22). The timing register 19 inthe GPS module 12 can be triggered via the special hardware (external)connection, and once so triggered, will store the current reading of theGPS local clock (oscillator) 18. Frame edges are not synchronized withGPS PVT calculations; in other words, the GPS module cannot be made toperform a PVT calculation at the instants of time at which frame edgesfrom the base station arrive at the GPS receiver (cellular module 11),and so what might be called frame edge time is different from what mightbe called GPS PVT calculation time.

As a result of these constraints, several observations can be made.First, only the downlink data frames can be time-stamped. Second, inorder to carry out data frame time-stamping with GPS time, the GPSmodule must be flexible, i.e. it must provide GPS time for instants oftime at which triggers are received, instead of controlling the triggermodule to provide triggers at instants of time at which PVT solutionsapply. Third, because frame edges are not synchronized with a GPS PVTcalculation, there must be a way to measure the time difference betweenthe instant a frame edge (Frame number) is time stamped and the instantto which the (latest) PVT calculation applies.

What the invention provides therefore is a way to time-stamp data framesin the cellular phone with GPS-time, given the above constraints andobserved consequences.

Referring now to FIG. 2 and also to FIG. 1, the method provided by theinvention is as follows. In a first step 31, a (downlink) frame istransmitted by the base station 24 according to a local clock (notshown). In a next step 32, the frame counter 20 of the cellular module11 detects the frame edge of the transmitted (downlink) frame andapplies a trigger to the special hardware connection 29 linking thecellular module to the GPS module 12. The trigger signals to the GPSmodule the arrival of the frame (edge) in the cellular module, causing,in a next step 33, the timing register 19 of the GPS module to recordthe current reading of the local clock 18. In a next step 34, the GPSmodule performs a PVT solution (using any of various known algorithms,such as a least squares algorithm, or navigation filters, such as anextended Kalman filter). In a next step 35, the time according to thelocal clock for which the PVT solution applies (i.e. the time at whichthe PVT solution is provided) is recorded by the GPS processor 21. Thenin a next step 36, the GPS processor reads the time held by the timingregister, which indicates, per local clock time, the instant of arrivalof the frame edge arriving just prior to obtaining a PVT solution. Thenin a next step 37, the GPS processor determines the difference in timebetween the time for which the PVT solution was obtained and the timewhen the frame edge arrived triggering the timing register. (The GPSlocal clock/oscillator 18 is typically a 50 MHz oscillator, giving aso-called jitter of only 6 m, determined from the formula 1/50×10⁶×c,where c is the speed of light). In a next step 38, the GPS time obtainedas a by-product of the PVT solution is corrected for the difference intime between the instant at which the PVT solution was provided and theinstant at which the trigger arrived in the GPS module, so as to obtainthe GPS time corresponding to the trigger, thus relating a frame edgeinstant of arrival (for a frame with a known frame number) to GPS time.In a next (optional) step 39, the above steps are repeated periodically,so as to obtain values for GPS time for several sequential frame edges,and the base station or the GPS receiver can then estimate the clockdrift for the base station clock (not shown) used in timing frametransmissions. In next step 40, the GPS module sends the GPS time forthe instant it received the trigger to the cellular module (along with,optionally, a corresponding position determination, as described below),and the cellular sends the information to the base station in an uplinkframe.

Referring again to FIG. 1, it should also be understood that theinvention comprehends using the PVT solution determined by the rangingcomponent (12) (FIG. 1) for calculating the propagation delay betweenthe ranging receiver 10 and the base station serving the cellularcomponent 11. Preferably, the GPS processor 21 adjusts the position ofthe ranging receiver 10 so as to correspond to the instant of time atwhich the trigger was received by the GPS module 12. Then, along withthe time mark the cellular component sends back to the base stationenabling the base station to synchronize to GPS time, the receiver canalso send back to the base station in the uplink frame the positiondetermined from the PVT solution (adjusted to the trigger time), asindicated in FIG. 1 and FIG. 2, for use by the base station incalculating propagation delay for signals sent to the ranging receivercellular component 11 from the base station (as an alternative to thebase station calculating propagation delay via a timing advanceprocedure, which provides only relatively poor accuracy, to within a fewmicroseconds, corresponding to a few hundreds of meters in position).

It should be understood that a base station need not provide atime-stamp for a time-stamped frame in order for the invention to work.A GPS receiver adapted to use the invention can provide the GPS time ofarrival of a frame edge of any frame (identified by its frame number)without using a time-stamp value for a time-stamped frame (edge) inarriving at a PVT solution. In the usual mode of operation, as explainedabove, the GPS receiver operating according to the invention isoperating in favorable signal conditions and so readily determines anaccurate PVT solution without having to rely on assistance by thecellular network. In general, however, a GPS receiver operatingaccording to the invention provides what amounts to a correction to atime-stamp value provided by a base station, the correction providing amore precise GPS time of arrival of the time-stamped frame. In otherwords, referring again to FIG. 1, in general, the GPS receiver 10receives both a time-stamp value frame, conveying an estimated GPS timefor a triggering frame (i.e. an estimate of when the triggering framewill arrive at the GPS receiver and be received by the cellular module11), and also the triggering frame (the time-stamped frame). It thendetermines a more precise GPS time for the time-stamped frame asdescribed above, and then sends the correction back to the base station24, in effect saying, “Hey base station, you sent me a frame edge thatyou said I would receive at X o'clock GPS time, but the time you gave mewas Y seconds behind the correct time.”

Use of a (cellular signal) frame counter (or a time slot counter, bitcounter, or, in a CDMA system, a chip counter, or other, similardevices), which would have to be provided as additional equipment in aGPS, is just one way to implement the present invention, and it is thepreferred embodiment for some applications, applications where it is notnecessary to have the highest possible accuracy. In case of GSM cellularcommunication, a frame counter can be used to generate a trigger pulseat a frequency of 216.68 Hz. It is also possible to generate triggerpulses at the time slot rate (there being eight time slots per frame)using a time slot counter; use of the time slot rate would give atrigger pulse at a frequency of 1733.4 Hz. It is possible to generate atrigger pulse at an even still higher frequency; using the bit rate156.25 bits per time slot would give a trigger pulse at a frequency of270844.17 Hz (i.e. using a data bit counter instead of the frame counterwould in the case of a GSM cellular communication signal provide atrigger pulse at a frequency of 270844.17 Hz). In other cellular systemsbesides GSM, data rates and data structures are of course different, andthe invention is not intended to be restricted to GSM cellularcommunication.

To implement the present invention, a typical GPS receiver would have tobe modified to include a cellular module with at least the functionalityindicated above, as well as hardware for providing the trigger pulse andthe hardware associated with the clock register.

There are other uses for an absolute time reference besides assistingGPS receivers in recovering GPS time. Absolute time can also be used tomeasure drift of cellular base station clocks with respect to GPS time(as noted above), and to measure time differences between neighboringbase stations. Such measurements are needed for example in network-basedpositioning methods including Enhanced Observed Time Difference (E-OTD)and Time of Arrival (TOA) positioning methods.

Note that in case of the base station providing an initial estimate ofGPS time for a trigger, it is not necessary that the cellular module 11construct the time-mark message. The cellular module in someapplications sends only a “Register triggered with Frame No. 1022”message to some other entity, which may reside outside of the GPSreceiver or may be the GPS module, and that other entity calculates thetime-mark.

Note also that although the invention has been shown and described inthe preferred embodiment in which the GPS module 12 includes a (local)clock 18, in other applications the clock used by the GPS module toprovide (local) time is located outside of the GPS module. For example,the clock is in some applications shared with some another entity suchas the cellular module. The clock used by the GPS module can even resideoutside of the GPS receiver.

It should also be noted that the invention is of use in providing abasis for setting the clock used by a module which is other than a GPSmodule. The invention should be understood to be of use in setting theclock used by any application that requires some version of universaltime, and in particular, other ranging receiver applications besides GPSreceivers, including for example ranging receivers for use with theRussian-Global Navigation Satellite System (GLONASS) positioning system.

It is to be understood that the above-described arrangements are onlyillustrative of the application of the principles of the presentinvention. Numerous other modifications and alternative arrangements maybe devised by those skilled in the art without departing from the scopeof the present invention, and the appended claims are intended to coversuch modifications and arrangements.

1. An apparatus (10), comprising: a) a cellular module (11), responsiveto a portion of a downlink frame signal conveying a cellular frameprovided by a cellular communication system; and b) a main module (12),responsive to ranging signals from beacons (27) of a positioning systemin which time is kept according to a predetermined time reference; theapparatus (10) characterized in that: the cellular module (11) providesa trigger signal indicating the arrival of the portion of the downlinkframe signal, and is further responsive to a signal indicatinginformation for relating the instant at which the trigger signal isprovided to a time value according to the predetermined time reference;the apparatus (10) relates the instant at which the trigger signal isprovided by the cellular module (11) to a time value according to thepredetermined time reference; and the main module (12) uses the timevalue for synchronizing into ranging signals from beacons (27) of thepositioning system; wherein the trigger signal is provided over aconnection (29) between the cellular module (11) and the main module(12) so constructed that any delay between sending and receiving thetrigger signal is either negligible or can be determined.
 2. Theapparatus of claim 1, further characterized in that the cellular module(11) provides to a base station of the cellular communication system theinformation for relating to a time value, according to the predeterminedtime reference, the instant at which the trigger signal is provided bythe cellular module (11) to the main module (12).
 3. The apparatus ofclaim 2, further characterized in that the main module (12) provides aposition for the instant when the trigger signal is received by the mainmodule (12), and the cellular module (11) is responsive to the position,and provides the position to the base station.
 4. A system, comprisingan apparatus according to claim 2, and further comprising the beacons(27) of the positioning system that provide the ranging signals and alsocomprising the base station receiving the information for relating to atime value, according to the predetermined time reference, the instantat which the trigger signal is provided.
 5. A system, comprising anapparatus according to claim 1, and further comprising a cellular basestation (24), for providing the cellular frame.
 6. A system, comprisingan apparatus according to claim 1, and further comprising the beacons(27) of the positioning system that provide the ranging signals.
 7. Amethod for having a ranging receiver (10), operative in communicationwith a positioning system and also operative in communication with acellular communication system, provide information to a base station ofthe cellular communication system of use in enabling the base station tosynchronize to time according to the positioning system, the rangingreceiver (10) including a cellular module (12) adapted for communicationwith the cellular communication system and also including a main module(12) adapted for communication with the positioning system, the methodcharacterized by: a) a step (32) in which the cellular module (11)detects a portion of a downlink frame signal and provides a triggersignal to the main module (12) via a connection so constructed that anydelay between applying a signal and receiving the signal is eithernegligible or can be determined; b) a step (40) in which the cellularmodule (11) receives a signal indicating information for relating theinstant at which the trigger signal is provided to a time valueaccording to the predetermined time reference; and c) a step (40) inwhich the apparatus (10) relates the instant at which the trigger signalis provided by the cellular module (11) to a time value according to thepredetermined time reference.
 8. The method of claim 7, furthercharacterized by a step (40) in which the cellular module (11) providesto the base station the information for relating to a time valueaccording to the positioning system, the instant at which the triggersignal is provided.
 9. The method of claim 8, further characterized by astep (38 40) in which the main module (12) also provides a position forthe instant when the trigger signal was received by the main module(12), and the cellular module (11) is responsive to the position, andprovides the position to the base station.
 10. An apparatus (10),comprising: a) a cellular module (11), responsive to a portion of adownlink frame signal conveying a cellular frame provided by a cellularcommunication system; and b) a main module (12), responsive to rangingsignals from beacons (27) of a positioning system in which time is keptaccording to a predetermined time reference; the apparatus (10)characterized in that: the cellular module (11) provides a triggersignal indicating the arrival of the portion of the downlink framesignal; and the apparatus (10) relates the instant at which the triggersignal is provided by the cellular module (11) to a time value accordingto the predetermined time reference; wherein the cellular module (11)provides to the cellular communication system information about therelationship of the downlink frame signal to a time value according tothe predetermined time reference.
 11. The apparatus of claim 10, furthercharacterized in that the cellular module (11) provides to a basestation of the cellular communication system the information forrelating to a time value, according to the predetermined time reference,the instant at which the trigger signal is provided by the cellularmodule (11) to the main module (12).
 12. The apparatus of claim 11,further characterized in that the main module (12) provides a positionfor the instant when the trigger signal is received by the main module(12), and the cellular module (11) is responsive to the position, andprovides the position to the base station.
 13. A system, comprising anapparatus according to claim 11, and further comprising the beacons (27)of the positioning system that provide the ranging signals and alsocomprising the base station receiving the information for relating to atime value, according to the predetermined time reference, the instantat which the trigger signal is provided.
 14. A system, comprising anapparatus according to claim 10, and further comprising a cellular basestation (24), for providing the cellular frame.
 15. A system, comprisingan apparatus according to claim 10, and further comprising the beacons(27) of the positioning system that provide the ranging signals.
 16. Amethod for having a ranging receiver (10), operative in communicationwith a positioning system and also operative in communication with acellular communication system, provide information to a base station ofthe cellular communication system of use in enabling the base station tosynchronize to time according to the positioning system, the rangingreceiver (10) including a cellular module (12) adapted for communicationwith the cellular communication system and also including a main module(12) adapted for communication with the positioning system, the methodcharacterized by: a) a step (32) in which the cellular module (11)detects a portion of a downlink frame signal and provides acorresponding trigger signal to the main module (12) via a connection soconstructed that any delay between applying a signal and receiving thesignal is either negligible or can be determined; b) a step (40) inwhich the apparatus (10) relates the instant at which the trigger signalis provided by the cellular module (11) to a time value according to thepredetermined time reference; and c) a step (40) in which the cellularmodule provides to the cellular communication system information aboutthe relationship of the downlink frame signal to a time value accordingto the predetermined time reference.
 17. The method of claim 16, furthercharacterized by a step (38 40) in which the main module (12) provides aposition for the instant when the trigger signal is received by the mainmodule (12), and the cellular module (11) is responsive to the position,and provides the position to the base station.